This invention relates to a material having liquid crystal droplets dispersed in a copolymer matrix, and, more particularly, to such a material prepared by photopolymerization.
Liquid crystals are materials which are liquids in the conventional sense of being a condensed state that is flowable. They contain long molecules of particular structures which exist in an oriented arrangement wherein many molecules are aligned with respect to each other. In a basic liquid crystal cell used in display devices, liquid crystals are placed between two sheets of a transparent conductor material, so that the liquid crystals are oriented in a selected manner. Upon application of an electric field, the liquid crystal molecules reorient in another manner. This reorientation is used to control the transmission of the cell to polarized light passed through the cell.
The basic liquid crystal cell just described is operable with many types of liquid crystal materials, but the design of the cell is cumbersome to use in some applications. The cell can be used to control polarized light, so that a polarizer is required. Since the liquid crystal remains a liquid within the cell, leakage of the liquid crystal material from the cell, or introduction of contaminants into the cell, can significantly interfere with the operation of the device. The preparation of curved or irregularly shaped displays, and the mass production of displays, is also difficult with this approach. Often, cost and weight considerations make this type of design inappropriate for particular applications.
Another approach to controlling the transmission of a cell to light is to divide the liquid crystal material into small droplets positioned between the transparent electrodes. If a liquid crystal is segregated into discrete droplets, the orientations of molecules within adjacent droplets are not correlated. However, the alignment of molecules in previously randomly oriented droplets can be achieved by an electric field applied to the transparent electrodes.
A different contrast mechanism operates when the liquid crystal molecules are provided in small droplets rather than in a continuous thin film between the electrodes, so that the cell can control light that is not previously polarized. When the molecules in the various droplets of a liquid crystal material are randomly oriented with respect to those in adjacent droplets, the liquid crystal scatters and does not transmit the light. Upon alignment of the molecules in the adjacent droplets, incident light is more readily transmitted through the bulk of the liquid crystal in the direction of the long axis of the liquid crystal molecules. This contrast mechanism depends upon scattering of light, not upon polarization effects, and is therefore useful with unpolarized light.
In one type of device, droplets or bubbles of the liquid crystal material are dispersed through the matrix of a transparent solid to form a composite material. The liquid crystal material and the transparent solid are selected to have matched indices of refraction. The liquid crystal material within the droplets is sealed against leakage and intrusion of contaminants. The droplets of liquid crystal are typically about 0.2 micrometer or greater in size and of irregular size and shape distribution, and often dispersed throughout the solid in a somewhat irregular distribution. Nevertheless, the solid matrix with the encapsulated droplets of liquid crystal experiences the same transition in optical transparency when an electrical field is applied, and can therefore be used in displays. Further, the use of plastic substrates that are coated with an optically transparent conductive layer and containing the composite material would allow for the preparation of large sheets which could be cut and shaped as needed.
Although such encapsulated liquid crystal materials are useful, their commercial use is inhibited by a lack of controllability in preparation and processing of the liquid crystal composite material. Processing usually requires heating of the encapsulated material, a step that is somewhat difficult to control. The droplets of liquid crystal material may be irregularly shaped and sized, and the liquid crystal material may vary from region to region in its response to an applied voltage.
There therefore exists a need for an improved technique for preparing liquid crystals encapsulated within a matrix to form composite materials for use in displays and other devices. The present invention fulfills this need, and further provides related advantages.